This invention relates to cascaded absolute encoding apparatus and in particular to a programmed combining of phased encoders to increase the accuracy of the read-out.
Digital controls and computing systems are widely employed in industry and related arts. Digital systems, of course, require conversion of analog-type data into digital form for incorporation into the logic and computing systems. Although conversion systems and devices are available, digital output transducers which produce a digital output in accordance with the phenomena being measured are preferably employed to minimize the complexity of the processing system, increase the accuracy, and the like. For example, rotary shaft encoders are widely employed to directly convert rotary and/or linear motion and position to a related digital signal output. A practical rotary absolute encoder may include a rotating element such as a disc mounted to the shaft and located to pass between an energy source and an energy detector. The disc is mounted with appropriately spaced energy opaque and transparent portions circumferentially arranged about the axis of rotation such that the output of the detector means is a series of pulse signals indicating the position of the shaft. By employing a plurality of detector means, the accuracy of the position can be detected within the accuracy of the smallest opaque portion of the least significant area. For example, in such a device a disc having a plurality of concentric tracks or rings, each of which includes equicircumferentially distributed adjacent light transparent and opaque sections or spaces may be coupled to a shaft with a light source mounted to one side of the disc and a photosensitive means mounted to the opposite side of the disc. The photosensitive means includes pick-up devices such as photocells oriented or aligned to separately respond to each of the concentric tracks. The successive tracks develop interrelated signals with the number of opaque sections and photosensitive devices increasing with the number of tracks. For example, a single photocell and a single track having two equal and opposite sections produces an indication of the shaft position within one half revolution. The use of two photocells requires two tracks of a similar 180.degree. characteristic but off set by 90.degree. and provides resolution within one quarter of the shaft position. A third track and photocell would require four sections and produce a resolution within one eighth. Additional tracks and photocells are similarly interconnected into the system to provide an increase in the reading of the shaft position. A ten channel encoder would thus provide a shaft indication within 1/1024 of a one shaft revolution. An absolute encoder employing a plurality of concentric rings generates a binary output, generally identified as a reflective or gray code, in which a single bit change occurs between any two successive position outputs. The output number is an exponent equal to the number of rings to the base two.
Well known electronic logic hardware is connected to the photosensitive devices to generate a numerical reading or output of the shaft position. Such encoders are employed for monitoring of machine position by suitable connecting of the encoder shaft to the appropriate machine shaft. The shaft can, of course, be coupled by the conventional gear, chain, pulley, or similar accurate drive positioning.
As the number of tracks and photocells increase the alternating opaque and transparent sections become successively smaller. As a practical matter there are limits on the size and therefore resolution which can be employed in any per unit of track length. Obviously the resolution can be increased by increasing the size of the disc or carrier. There are, of course, practical limits on such a development.
A further way of increasing the resolution or accuracy is to employ a plurality of cascaded encoders in such a manner that each successive encoder moves or advances a single bit for each complete revolution of the immediately preceeding encoder. This can be readily accomplished by the inter connection of the encoder shafts to each other through appropriate gears, sprockets, belts, chains or the like such that the corresponding rotational relationship is developed. Thus, by reading the position of the several encoders, the machine component position is determined to an accuracy related to the multiple of the position read-out of both encoders. For example, a pair of identical encoders each capable of indicating 1024 discreet positions when cascaded and properly interconnected, provides a read-out which is a multiple of the two, or 1,048,575 positions. Although such high resolution encoders can be constructed, the interconnection between them must be to a high degree of accuracy such that the several rotating discs simultaneously pass through the zero settings, or an erroroneous reading results. As a practical matter, this requires the use of very special coupling systems and essentially eliminates the possibility of using commercially produced and relatively inexpensive production gears, belts, chains and other similar coupling devices. As a result, high resolution encoding apparatus is relatively expensive and even then such devices are not readily available. There is, therefore, a need for a simple, reliable and relatively inexpensive high resolution encoding apparatus.